What We Know About the Dangerous COVID B.1.617.2 (Delta) Variant



The B.1.617.2 coronavirus variant originally discovered in India last December has now become one the most — if not the most — worrisome strain of the coronavirus circulating globally. Recent research suggests it may be the most transmissible variant yet and has fueled numerous waves of the pandemic around the world. B.1.617.2 has already spread to at least 62 countries. The strain undoubtedly contributed to the massive wave of cases that has inundated India in recent months, and makes up more than 90 percent of new cases in the U.K. and about 10 percent of new infections in the U.S. On top of that, it may be more likely to infect people who are only partially vaccinated than other strains. Below is what we know about B.1.617.2 — also known as the Delta variant.

How is B.1.617.2 different from other variants, and why may it be more dangerous?

The Delta variant has multiple mutations that appear to give it an advantage over other strains. The most important apparent advantage is that the mutations may make the strain more transmissible, which would also make it the most dangerous variant yet. One study indicated B.1.617.2 may be up to 50 percent more transmissible than the B.1.1.7 (U.K./Alpha) variant. Professor Neil Ferguson, a leading epidemiologist at Imperial College London and one of the chief pandemic advisers to the U.K. government, said on June 4 that the “best estimate at the moment” is that Delta is 60 percent more transmissible than Alpha, which is itself more transmissible than the original strain of the coronavirus that emerged in China in late 2019 — and that is why scientists believe it became a dominant variant globally. As a wave of infections hit the U.K. in May, Britain’s health minister said the Delta variant was estimated to be 40 percent more transmissible than the Alpha variant.



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